The processing of polyolefins and other polymers into films, sheets or tubes usually takes place by extrusion. In this process, the polymer is melted in an extruder and forced through a nozzle into the desired shape.
For economic reasons, the highest possible throughput through the extruder is typically desired. One way of increasing the throughput of an extruder is to increase the speed of the screw. However, the viscoelastic behavior of the polymer melt typically sets limits in this process which are well below the values which can be theoretically achieved by the machine. More specifically, if the extrusion rate exceeds a value that is dependent on the polymer to be processed, defects known as melt fracture occur in the surface of the extruded material.
Although those skilled in the art differentiate between various types of melt fracture, the characteristic of concern is a matte, rough surface of the extrudate, also known as “sharkskin”. Several measures have been proposed for prevention of this undesired melt fracture. Another important melt fracture mechanism is gross melt fracture, which take the form of severe irregular distortions at higher throughput rates at which sharkskin effects are seen.
As temperature is increased, melt fracture is displaced towards higher shear rates. Accordingly, it is possible to increase temperature to move to higher sustainable shear rates. However, this method can only be employed to a limited extent. Increasing processing temperatures increases operation costs, produces heat dissipation problems and can result in discoloration and thermal degradation of the polymer.
Another possibility involves modifying the extruder nozzle geometry. Nozzles having a relatively large cross-section produce a relatively high material throughput at the same shear rate. This measure is employed typically in the production of monoaxially or biaxially stretched films in which the relatively thick film obtained due to the enlargement of the nozzle slit can be adjusted to the desired thickness by means of a relatively large stretching ratio. Of course, this process cannot be used in all applications.
Another way of avoiding melt fracture comprises modifying the viscoelastic behavior of the polymer melt by means of additives. Examples of such additives, which arc generally compatible with the polyolefin include low-molecular-weight polyethylene waxes. However, highly compatible additives may adversely affect the properties of the polymer (e.g., tear resistance).
Another method involves modifying the interactions of the polymer melt with the nozzle walls by means of suitable additives. Such additives are generally not compatible with the polymer. They migrate to the boundary layer between the polymer melt and the nozzle wall where they act as lubricants.
It is known to use specific fluorine-containing polymers, and in particular fluroelastomers, as processing auxiliaries for polyolefins (see U.S. Pat. No. 3,125,547). These fluorine-containing polymers are generally obtained from vinylidene fluoride, vinyl fluoride, hexafluoropropene, chlorotrifluoroethylene or tetrafluoroethylene. However, further improvement in the flow behavior continues to exist in the art.
Furthermore, linear polyolefins, such as linear polyethylene, blended with fluoroelastomer processing aids in particular can be especially difficult to melt process, as compared to highly branched polyethylenes. In particular, while addition of fluoroelastomer may be effective to reduce sharkskin effects, such additives typically do not have an impact on the onset of gross melt fracture.
Increasing motor load, head pressure and/or torque can place undesirable, unacceptable, or unattainable requirements on specific machinery. For example, a given extruder having a specific motor power and gearing will reach a maximum of motor load or head pressure under certain melt temperature conditions for a given polymer being processed. If a polymer or polymer blend is introduced to such an extruder having a higher requirement for power in at least one component, such as a polymer having higher molecular weight and/or narrower molecular weight distribution and/or lower shear sensitivity, the extruder will reach a maximum of one or several of these parameters and, consequently, be limited in its ability to pump/perform at a similar level to the performance expected/demonstrated with a highly branched or broader molecular weight distribution polymer. Even highly adjustable, high performance melt processing machinery, that can withstand the higher loads, consumes more power in processing the aforementioned linear polyethylenes.
Linear polyethylenes and elastomeric blends thereof may exhibit other imperfections during extrusion (specifically blown film extrusion) that may be undesirable, such as gross melt fracture and/or sharkskin effects (discussed supra). Melt fracture can have a deleterious effect on optical properties and/or physical properties of the film. Typically, when a linear or high molecular weight thermoplastic resin is extruded through a die, smooth extrudates can only be obtained up to a certain shear stress (i.e., shear rate; extruder output rate). Beyond that, melt fracture becomes a significant problem. Therefore, it would be desirable to identify process aids that delay, suppress or eliminate the onset of melt fracture to enable operation at higher shear rates or output rates without surface defects, non-uniform cross-sectional thickness polymer decomposition (charring), or die drool.
In the case of polyolefin resins, even the employ of the above described process aids falls short when it comes to the industry's appetite for ever increasing production rates. This quest for higher extrusion rates, which is driven primarily by economic considerations related to the efficient use of processing equipment and human resources, is of particular interest to producers of melt processable polymer products, for example, blow film, blowmolded products, etc.
As the demand for high performance plastics continues to grow, new and improved methods of providing superior product more economically are needed to supply the market. In this context, various polymer process aids and process improvements are constantly being evaluated; however, the identities of improved and/or additional effective process aids continue to elude the industry. Consequently, a long felt, yet unsatisfied need exists for new and improved methods and compositions for processing polymers.